Induction coil for zone melting of semiconductor rods

ABSTRACT

An induction coil is provided for the floating zone melting of semiconductor rods, the coil being formed of aluminium tubular material having an exterior coating of anodically formed aluminium oxide. The ends of the coil are received in air tight, conductive relation within bores provided in a pair of terminal blocks, and the space surrounding the coil ends within the bores is filled with solder.

United States Patent [1 1 Baumgartner et a1.

[ INDUCTION COIL FOR ZONE MELTING OF SEMICONDUCTOR RODS [75] Inventors: Werner Baumgartner; Norbert Schink; Manfred Schnoeller; Ludwig Sporrer, all of Munich, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & Munich, Germany 122 Filed; Nov. 10, 1972 21 Appl. No.: 305,379

[30] Foreign Application Priority Data Nov. 24, 1971 Germany 2158274 [52] US. Cl. 219/1079, 219/1043 [51] Int. Cl. H051) 5/08 [58] Field of Search 219/1079, 10.67, 10.43,

219/1069, 10.73; l3/D1G. 1

[56] References Cited UNITED STATES PATENTS Keller et a1. 219/1067 x 1 Apr. 2, 1974 2,446,202 8/1948 Vang 219/1043 X 3,403,241 9/1968 Kaufiman 219/1079 X 3,363,079 l/l968 Schroyer 219/1079 X 3,488,236 1/1970 Van Husen 219/1079 X Primary Examiner-Bruce A. Reynolds Attorney, Agent, or FirmHill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson [57] ABSTRACT An induction coil is provided for the floating zone melting of semiconductor rods, the coil being formed of aluminium tubular material having an exterior coating of anodically formed aluminium oxide. The ends of the coil are received in air tight, conductive relation within bores provided in a pair of terminal blocks, and the space surrounding the coil ends within the bores is filled with solder.

9 Claims, 2 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to induction coils and more particularly to such coils which are employed with apparatus for purifying semiconductor materials by the floating zone meltingprocess.

2. The Prior Art Floating zone melting is a process used during manufacture of semiconductor materials to produce purified rods of semiconductor material. A localized high temperature zone is formed within a rod-shaped member formed of the semiconductor material to be treated, and this zone is gradually moved from one end of the rod to the other. By means of the floating zone melting process, impurities are transported to one end of the rod, leaving the remaining material more free of these impurities. The localized heating is carried out by use of a high frequency induction coil, and the zone is moved relative tothe'semiconductor rod, by moving the rod endwise through the coil.

The zone melting process is preferably carried out in a controlled atmosphere, such as in a vacuum or in a protective atmosphere of nonreactive gasses. Typical gasses which may be used are hydrogen, argon or other inert gasses. At times, in order to protect the purity of a gaseous atmosphere during floating zone melting, a pressure greater than atmospheric is maintained within a closed vessel containing the apparatus needed to carry out the process, which insures that if there is any leakage, it is from the outside into the interior of the container. When this process is used, and especially when argon is used as a protective gas, electric sparkovers sometimes occur between convolutions of the induction heating coil. These spark-overs have the very damaging effect on the quality of the semi-conductor material being treated. It is therefore desirable to provide an, arrangement which is not as subject to sparkovers as the coils employed in the fast.

SUMMARY OF THE INVENTION It is a principalobject of the present invention to provide an induction heating coil which is not subject to spark-overs between convolutions of the coil.

Another object of the present'invention is to provide a coil in which the coil surface, iseasily cleaned from vapor deposited impurities which occur during use.

Afurther object of the present invention is to provide apparatus. for mounting such an induction coil, which insures good. electrical contact and an air-tight connection between. the coil and the mounting.

These, and other objects and advantages of the present invention willlbeeome manifest; by. an'inspection of the following description and the accompanyingdrawings.

In one embodiment of the present invention an induction coil is provided which is formed. ofa plurality. of convolutions of hollow aluminium tubing, such tub ing having an electtrically insulating coating formed of aluminium oxide covering, its. exterior, the ends of the coil being received by metal'terminalbloeks. inelectrically conductive and air-tight relationship.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the accompanying drawings in which:

FIG. 1 is a perspective view of an illustrative embodiment of the present invention;

FIG. 2 is a plan view, partly in cross-section, of an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I an illustrative embodiment of the present invention is shown in which the induction coil 1 is formed of a plurality of convolutions of hollow aluminium tubing, a straight section of the tubing is joined to each end of the coil, and the free ends of the straight sections are received in bores 4 and 5, provided within metal tenninal blocks 6 and 7, respectively. The bores 4 and 5 are preferably just large enough to receive the tubing ends. A recess 8 is provided at the end of the bore 4 which opens toward the coil 1 and a similar recess 9 is provided at the end of the bore 5. The recesses 8 and 9 are preferably formed by counterboring the blocks 6 and 7.

The blocks 6 and 7 are both formed of aluminium, and the recesses 8 and 9 are filled with aluminium solder after the ends of the coil 1 have been inserted into the bores 4 and 5. The aluminium solder, after being heated to approximately 570 C, and allowed to cool, forms a firm bond betweenthe blocks 6 and 7 and the ends of the coil 1, establishing both good mechanical and electrical contact therebetween.

The aluminium surface of the coil 1 is anodized to provide a surface coating of aluminium oxide with a thickness of between 10 and 200 microns. The anodizing of the coil 1 is performed in a conventional way with the terminal blocks 6 and 7 being connected to a source of electric potential during the anodizing process. The blocks 6. and 7 are connected to a conventional mounting arrangement, having means for supporting the coil 1 during the floating zone melting process, and means for passing an electrical current through it. The mounting arrangement preferably includes a brass member, plated with silver for establishing electrical contact with each of the terminal blocks 6 and 7.

FIG. 2 shows an alternative embodiment of the present invention. In this embodiment the terminal block 16 is formed of silver and is provided with a bore 14 just large enough to receive an end section 12 of the coil 11. A second identical terminal block (not shown) is provided for connection to the other end of the coil 11. The bore 14 is preferably sized to receive the end section 12 in tight-fitting engagement, and optionally may be provided with screw threads which cooperate with threads on the exterior of the end section 12. At the end of the bore 14 toward the coil 11 a counter bore is provided, so that a space exists between the outer wall of the end" section 12 and the surface of the counter bore. This space is preferably filled with molten tin solder 17', which, upon cooling, forms a firm bond. During'the soldering, a steel tube (not shown) is preferably inserted into the interior of the end section 12 to lend mechanical rigidity and to assist in dissipating heat-during soldering. For this purpose, the steel insert is preferably of nearly the same size as the interior diameter of the end section 12 so that the inserted portion is in contact with the interior of the end section 12 over substantially all of its surface. When the solder is placed in the bore 14 the terminal block 16 is preferably heated to the melting point of the solder so that the solder can run freely into the space desired to be filled, and to allow the blocks toshrink, on cooling, the same amount as the solder. Thereafter, the end section 12 is also heated, and twisted within the silver block 16. This provides for additional electrical contact between the end section 12 and the terminal block 16.

The steel rod which is inserted into the end 12 of the coil 11 during the soldering process, is thereafter removed. The mounting arrangement supporting the terminal block 16, as with the structure of FIG. 1, is preferably formed of silver coated brass and is constructed in the conventional manner.

From the foregoing it will be-appreciated that the coil arrangement of the present invention achieves the objects of providing an induction coil apparatus which is less subject to'spark-overs. The insulating coating of anodically formed aluminium oxide on the surface of the coils 1 and 11 gives a high resistance to sparking between the convolutions of a coil in any atmosphere, and the mounting arrangements which have been described assure that the coil is in good mechanical and electrical contact with its terminal blocks. The aluminium oxide surface on the coils of the present invention permit easy removal of any residue deposited on the coils during operation. As the mechanical connections between the ends of the coils l and 11 are air tight, the terminal blocks may be incorporated into the wall of the vessel in which the zone melting process takes place, with no danger of a leak through the joint between the coil and the terminal blocks, permitting the maintenance of a vacuum or other controlled atmosphere within the ves- SCI; I i

We claim as our invention:

. 1. An induction heating coil having a winding for the zone melting of semiconductor rods in a controlled gas atmosphere and a pair of aluminum terminal blocks, each for supporting an end of the coil, said coil consisting of a bent aluminum tube having a coating of electrically insulating anodically applied aluminum oxide on its surface, each of said blocks havinga bore therein for receiving an end of said coil in vacuum tight relationship. t

2. An induction heating coil according to claim 1, wherein each of said blocks is provided with a counter bore at the end of said bore in which a coil end is received and a quantity of aluminum solder disposed in said counterbore surrounding said coil end to form a vacuum tight connection.

3. A method of producing the coil according to claim 1, comprising the steps of forming a coil having a plurality of convolutions of aluminum tubing, inserting an end section of said coil into a bore provided within a terminal block, filling the space within said bore surrounding said end section with conductive material, and anodizing the exterior surface of said coil.

4. The method according to claim 3, wherein said bore passes through said terminal block, and including the step of inserting a rod of thermally conductive metal into the interior of said end section during said filling step.

5. The method according to claim 3, wherein said terminal block is formed of aluminum and said conductive material is aluminum solder, including the step of heating said solder to 570 C.

6. The method according to claim 3, wherein said terminal block is formed of silver and said conductive material is tin solder, including the step of heating said block while said solder is molten within said space.

7. The method according to claim 3, including the step of twisting said end section while said solder is molten within said space.

8. An induction heating coil having a winding for the zone melting of semiconductor rods in a controlled gas atmosphere, a pair of silver terminal blocks, each for supporting an end of said coil, said coil consisting of a bent aluminum tube having an electrically insulating coating of anodically applied aluminum oxide applied to its surface, said terminal blocks each having a bore for receiving one of said coil ends.

9. Apparatus according to claim 8, including a quantity of tin solder disposed in said bore surrounding said coil end. 

2. An induction heating coil according to claim 1, wherein each of said blocks is provided with a counter bore at the end of said bore in which a coil end is received and a quantity of aluminum solder disposed in said counterbore surrounding said coil end to form a vacuum tight connection.
 3. A method of producing the coil according to claim 1, comprising the steps of forming a coil having a plurality of convolutions of aluminum tubing, inserting an end section of said coil into a bore provided within a terminal block, filling the space within said bore surrounding said end section with conductive material, and anodizing the exterior surface of said coil.
 4. The method according to claim 3, wherein said bore passes through said terminal block, and including the step of inserting a rod of thermally conductive metal into the interior of said end section during said filling step.
 5. The method according to claim 3, wherein said terminal block is formed of aluminum and said conductive material is aluminum solder, including the step of heating said solder to 570* C.
 6. The method according to claim 3, wherein said terminal block is formed of silver and said conductive material is tin solder, including the step of heating said block while said solder is molten within said space.
 7. The method according to claim 3, including the step of twisting said end section while said solder is molten within said space.
 8. An induction heating coil having a winding for the zone melting of semiconductor rods in a controlled gas atmosphere, a pair of silver terminal blocks, each for supporting an end of said coil, said coil consisting of a bent aluminum tube having an electrically insulating coating of anodically applied aluminum oxide applied to its surface, said terminal blocks each having a bore for receiving one of said coil ends.
 9. Apparatus according to claim 8, including a quantity of tin solder disposed in said bore surrounding said coil end. 